Plant Reproduction and Multiplication. 1. Sexual Reproduction 2. Asexual Reproduction 3. Micropropagation

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1 Plant Reproduction and Multiplication 1. Sexual Reproduction 2. Asexual Reproduction 3. Micropropagation

2 Sexual Versus Asexual Propagation Sexual propagation - plants are propagated from seeds - gives rise to a seedling population Asexual (vegetative) propagation - plants are propagated from vegetative tissue or organs - gives rise to a clonal population

3 Sexual Reproduction

4 Reproductive Development in Angiosperms Dilkes et al Plant Cell. 16:

5 Embryo Development in Arabidopsis Laux et al. (2004) Plant Cell 16:S190-S202

6 Transition of Seed from Development to Germination Seed development Seed germination Fresh weig ght Maturation drying Cell expansion Histodifferentiation Time vivipary recalcitrant non-dormant orthodox dormant Vivipary and recalcitrant seeds start to germinate before completing the maturation drying stage Orthodox seeds dry to ~10% of moisture content towards the end of seed development Hartmann et al. (2002) Hartmann and Kester s Plant Propagation Principles and Practices 7 th ed. Prentice Hall, New Jersey, USA.

7 Early Stages of Seed Germination Some seeds germinate rapidly (non-dormant seeds) while others do not readily germinate (dormant seeds) Germination starts with the uptake of water by the dry seed and is completed when the radicle elongates and breaks through the layers surrounding the embryo a. Water uptake by imbibition b. Lag phase of germination c. Radicle protrusion Hartmann et al. (2002) Hartmann and Kester s Plant Propagation Principles and Practices 7 th ed. Prentice Hall, New Jersey, USA.

8 Abiotic Factors Influencing Seed Germination Temperature - temperature is positively correlated to germination rate - optimal temperature for non-dormant seeds: o C; some require 15 o C Water - water stress can reduce the rate of germination Air - O 2 is required for the respiratory process in germinating seeds - increased CO 2 in the soil may inhibit seed germination Light - some are light sensitive whereas others are light dependent Hartmann et al. (2002) Hartmann and Kester s Plant Propagation Principles and Practices 7 th ed. Prentice Hall, New Jersey, USA.

Epigeous germination (a): hypocotyl elongates, forms a hook and raises the cotyledons above the ground

9 Patterns of Seed Germination Hypocotyl is the stem section between the cotyledons and the radicle. Epicotyl is the section between the cotyledons and the first true leaves. Epigeous germination (a): hypocotyl elongates, forms a hook and raises the cotyledons above the ground Hypogeous germination (b,c): hypocotyl fails to elongate, only the epicotyl emerges above the ground, while cotyledons remain below ground

10 Definition of Seed Dormancy A dormant seed does not have the capacity to germinate in a specified period of time under any combination of normal physical environmental factors that are otherwise favourable for its germination [Baskin JM, Baskin CC. (2004) A classification system for seed dormancy. Seed Science Research 14: 1 16]

11 Classification of Seed Dormancy i. Physiological dormancy (PD) - most abundant and prevalent form - factors within the embryo that inhibit germination - requires treatments (scarification, after-ripening in dry storage, moist-chilling stratification, GA) to break dormancy - examples: A. thaliana, Helianthus annuus, Lycopersicon esculentum, Nicotiana spp. ii. Morphological dormancy (MD) - embryos are underdeveloped but differentiated - embryos are not (physiologically) dormant but simply need time to grow and germinate iii. Morphophysiological dormancy (MPD) - underdeveloped embryos which are physiologically dormant - requires treatments (warm and/or cold stratification, GA application) to break dormancy iv. Physical dormancy (PY) - caused by water-impermeable layers of palisade cells in the seed or fruit coat - requires mechanical or chemical scarification to break dormancy v. Combinational dormancy (PY + PD) - water-impermeable coats with physiologically dormant embryos Baskin JM, Baskin CC. (2004) A classification system for seed dormancy. Seed Science Research 14: 1 16

Basal - B1 to B4 (abundant endosperm and a tiny embryo) ii. Axile LA (linear axile embryo) iii. Peripheral - P iv. Dwarf seeds - MA v.

12 Morphology of Embryo and Endosperm in Angiosperm Seeds ND = non-dormancy PD = physiological dormancy MD = morphological dormancy MPD = morphophysiological dormancy PY = physical dormancy PY+PD = combinational dormancy Seed types: i. Basal - B1 to B4 (abundant endosperm and a tiny embryo) ii. Axile LA (linear axile embryo) iii. Peripheral - P iv. Dwarf seeds - MA v. Foliate axile - FA1 to FA4 (nutrient stored in cotyledon, little or no endosperm) [Martin (1946) American Midland Naturalist 36: ]: Finch-Savage & Leubner-Metzger (2006) New Phytologist 171 (3):

13 Seed Dormancy Release and Germination (a) Nicotiana sp. seed germination two steps: testa rupture followed by endosperm rupture Dormancy release and germination promotion occur during after-ripening dry storage (room temperature for several months) or via the light- GA pathway during imbibition - GA, ethylene and brassinosteroids (BRs) promote endosperm rupture - ABA inhibits endosperm rupture but not testa rupture (βglu I = class I β-1,3-glucanase) Brassica napus seed germination a one-step event: testa rupture plus initial radicle elongation Mature seed without endosperm - ABA does not inhibit testa rupture - ABA inhibits subsequent radicle growth Finch-Savage & Leubner-Metzger (2006) New Phytologist 171 (3):

14 Terminology After-ripening the loss of the dormant state over some period of time through exposure of the seeds to a set of environmental conditions after maturation and separation from the parent plant (Simpson, G.M Seed Dormancy in Grasses. New York: Cambridge University Press) Scarification The physical or chemical treatment given to some seeds in order to weaken the seed coat sufficiently for germination to occur ( Stratification Storing of seeds at low temperatures under moist conditions in order to break dormancy (

15 End of Lecture

DID YOU KNOW that the double coconut palm produces the largest

Seed Germination Processes and Requirements DID YOU KNOW that the double coconut palm produces the largest seed in the world? It weighs about 50 pounds. The smallest seeds are like dust. Whatever the size